What is wrong then the car jumps or feels as though it is skipping gears? on 2005 Pontiac Grand Prix

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2 answers
Engine misfire, need a tune up!
From the frequently reports problems page:

20 reports of Erratic Shifting of the Automatic Transmission
Verified

The transmission pressure control solenoid may fail causing erratic shifting. Our technicians tell us that partial dis-assembly of the transmission is necessary to replace a failed pressure control solenoid.